1. Technical Field of the Invention
The present invention relates to local loop technologies. More particularly, and not by way of any limitation, the present invention is directed to a multi-channel, multi-mode redundant optical local loop having a bus topology implemented with a pair of optical fibers.
2. Description of Related Art
As networks face increasing bandwidth demand and diminishing fiber availability in the existing fiber plant, network providers are migrating towards a new network technology called the optical network. Optical networks are high-capacity telecommunications networks comprised of optical and opto-electronic technologies and components, and provide wavelength-based services in addition to signal routing, grooming, and restoration at the wavelength level. These networks, based on the emergence of the so-called optical layer operating entirely in the optical domain in transport networks, can not only support extraordinary capacity (up to terabits per second (Tbps)), but also provide reduced costs for bandwidth-intensive applications such as the Internet, interactive video-on-demand and multimedia, and advanced digital services.
Of the several key enabling technologies necessary for the successful deployment of optical networks, Wavelength Division Multiplexing (WDM) technique has emerged as a crucial component for facilitating the transmission of diverse payloads regardless of their bit-rate and format over the optical layer. WDM increases the capacity of embedded fiber by first assigning incoming optical signals to specific wavelengths within a designated frequency band (i.e., channels separated by a predetermined spacing) and then multiplexing the resulting signals out onto a single fiber. Because incoming signals are not terminated in the optical layer, the interface is bit-rate and format independent, allowing service/network providers to integrate the WDM technology with existing equipment in the network.
By combining multiple optical signals using WDM, they can be amplified as a group and transported over a single fiber to increase capacity in a cost-effective manner. Each signal carried can be at a different rate (e.g., Optical Carrier (OC)-3, OC-12, OC-48, etc.) and in a different format (e.g., Synchronous Optical Network (SONET) and its companion Synchronous Digital Hierarchy (SDH), Asynchronous Transfer Mode (ATM), Internet Protocol (IP)-based data or multimedia, et cetera).
Current advances in WDM technologies allow a plurality of wavelengths to be multiplexed over a fiber using nanometer and sub-nanometer spacing (Dense WDM or DWDM). For example, up to 32 channels or carriers may be spaced 100 GHz apart (equal to 0.8 nm) in a multiplexed optical signal operating at around 1550 nm. In contrast, some of the standardized, xe2x80x9ccoarsexe2x80x9d wavelength separations include 200 GHz spacing (1.6 nm) and 400 GHz spacing (3.2 nm), both at around 1550 nm.
In order to fully utilize the capabilities of an optical network and to overcome what is commonly referred to as the xe2x80x9clast mile problemxe2x80x9d in the telecommunications art, optical access from end users, i.e., homes, offices, other buildings, etc., to the network is necessary. Whereas several access architectures such as Fiber To The Curb (FTTC), Fiber To The Home (FTTH), Fiber To The Building (FTTB) and the like have been proposed, the existing solutions for implementing such architectures are beset with various deficiencies and shortcomings.
For example, in current Passive Optical Network (PON) systems used for implementing an optical access network, an Optical Line Termination (OLT) unit is usually located in a local exchange and is connected to a plurality of Optical Network Units (ONUs) through a point-to-multipoint network comprised of fiber cables, splitters and other passive components. Thus, the PON access system is based on a xe2x80x9cstarxe2x80x9d topology which requires more fiber, particularly if redundancy is desired, thereby increasing the cost of the fiber plant. Moreover, any service upgrade beyond the initial capacity in an existing PON access system requires a major overhaul, causing service disruption and possibly widespread outage during system reconfiguration.
Another optical access solution available today is based on the ring topology. Those skilled in the art should readily appreciate that the optical ring topology a variant of the traditional Add/Drop Multiplexing (ADM) telephony technology and requires additional fiber to close the loop and provide an upstream data path. Consequently, the ring topology is also expensive in terms of the fiber plant needed.
Based upon the foregoing, it should be apparent that there has arisen an acute need for an access network solution that reduces the cost of deploying FTTC/FTTH/FTTB systems and fiber transmission infrastructure for an optical local loop. It would be of additional benefit to provide for the capability to detect fiber breakage or localized electro-optical failures in order to minimize or eliminate downtime caused thereby. Furthermore, it would be particularly advantageous if such a solution is capable of providing a transparent (i.e., non-service affecting) method for service upgrading on a per node basis.
Accordingly, the present invention advantageously provides an optical local loop having a bus topology with a pair of optical fibers disposed among a plurality of Optical Network Units (ONUs). One of the optical fibers provides a main or primary optical path while the other optical fiber is operable as a standby or secondary optical path. A Host Digital Terminal (HDT) is disposed at the head end of the optical local loop for concentrating optical signals for a network. A pair of receiver banks and a pair of transmitter banks are provided within the HDT for operating with the optical signals effectuated at a plurality of wavelengths which are multiplexed and de-multiplexed via a pair of Wavelength Division Multiplex (WDM) couplers disposed on corresponding optical fibers. The WDM coupler and associated optical fiber of an optical path may be optimized for operation around 1310 nm or 1550 nm bands. During transmission, the ONU nodes excite both optical fibers of the local loop and the received optical signals are monitored in the HDT for quality and integrity of the redundant optical paths. The monitoring mechanism is also operable to detect a fault or a potential fault associated with either of the optical paths. Further, such faults or potential faults can be isolated and localized for troubleshooting. A selectively actuatable mechanism provided in the HDT is operable to switch downstream transmission to the ONU nodes from one optical path to the other optical path of the local loop based on the path quality and integrity.
In one exemplary embodiment, each ONU is operable at a select wavelength and is comprised of a first circulator coupled to one optical fiber and a second circulator coupled to the other optical fiber. An optical filter is associated with each circulator for selecting the operating wavelength of the particular ONU. The receiving and transmitting units of the ONU are coupled to the circulators via an optical coupler having a 3 dB separation. In another exemplary embodiment, one or more ONUs are capable of operating at multiple wavelengths. A tunable filter is operably associated with each of the circulators in order to select among multiple wavelengths for a particular ONU node. These filters may be selected from the group consisting of fiber Bragg gratings, wavelength separators using interference filters and Fabry-Perot filters, et cetera.
In one aspect, the present invention is directed to an optical local loop comprising an HDT operable to transmit in a downstream direction on a pair of optical fibers coupled thereto, wherein at least one ONU node is coupled to the optical fiber pair. Preferably, the ONU node is operable to receive and transmit optical signals in at least one wavelength. A first receiver block disposed in the HDT is operable for receiving optical signals excited on a first fiber of the optical fiber pair. In analogous fashion, a second receiver block is disposed in the HDT for receiving optical signals on a second fiber of the optical fiber pair. A monitor is coupled to both first and second receiver blocks for monitoring the quality and integrity of the optical fiber pair based on the received optical signals emanating from the ONU nodes. Accordingly, protection switching may be effectuated transparently (i.e., without affecting or disrupting service) from one optical path to the other path when faults or potential problems are detected on a particular optical fiber path.
In another aspect, the present invention is directed to a method of evaluating integrity in an optical local loop arranged as bus having a pair of optical fibers, wherein a plurality of ONU nodes are disposed downstream from an HDT. During transmission, the ONU nodes are operable to transmit on both fibers of the local loop. A first receiver block in the HDT receives a first set of optical signals on a first optical path of the optical fiber pair, the optical signals being generated by the plurality of nodes disposed on the first optical path. Similarly, a second receiver block in the HDT receives a second set of optical signals on a second optical path of the local loop, wherein the second set of optical signals are generated by the same nodes disposed on the second optical path. Path quality and integrity are monitored by comparing the first and second set of optical signals, wherein loss of a particular wavelength from a select ONU node on one of the optical paths is indicative of fiber breakage or a localized electro-optical failure with respect to that path.